arXiv subjects

Khamphee Karwan

Publications and source records attributed to Khamphee Karwan.

At least 19 recordsLinked to original sources

arXiv2026

Spherical collapse and cluster number counts in DHOST theories that pass the constraints from gravitational waves

We investigate the spherical collapse model and the abundance of galaxy clusters in a class of degenerate higher-order scalar--tensor (DHOST) theories in which gravitational waves do not decay into scalar perturbations and which are consistent with current constraints from gravitational-wave observations. We find that deviations from Einstein gravity can become significant at late times when the background universe is close to the scaling regime during the matter-dominated epoch. These deviations suppress the growth of linear matter perturbations on small scales while increasing the extrapolated linear density contrast at collapse, obtained from the spherical collapse model. Using the analytic mass function, we compute the corresponding cluster number counts. The minimum mass threshold in the mass integration for each redshift bin is determined by matching the predicted number counts in the $Λ$CDM model with those inferred from the eROSITA survey. We find that the cluster abundance reaches its maximum at low redshift bin, and that the number of clusters in the highest redshift bin is suppressed as the deviation from Einstein gravity becomes larger. The parameters of the theory are chosen such that the deviation from Einstein gravity at present is consistent with the local astrophysical bounds from binary pulsar observations. We find that even under such strict constraints, the upper bound on the deviation leads to lower predicted number counts compared with the Poisson error of the eROSITA survey results. However, this may be a consequence of the uncertainties in computing the number counts for the DHOST theories using the spherical collapse model and the analytical mass function. In general, it may be concluded that the suppression of the cluster number counts is a consequence of the enhancement of the extrapolated linear density contrast.

astro-ph.CO↗

arXiv2026

Cluster number counts in dark energy model with energy and momentum coupling to dark matter

The influences on the cluster number counts from the coupling between dark energy and dark matter with momentum transfer are investigated. We find that the extrapolated linear density contrast computed from the spherical collapse model is suppressed when the strength of momentum transfer is increased. Using the Sheth-Tormen mass function, the cluster number counts are computed. The minimum mass limit in the mass integration for each redshift bin is determined by matching the predicted number counts from the $Λ$CDM model with the result from eROSITA surveys. We find that the number of clusters is maximal at a higher redshift bin, and the number of clusters in a maximum redshift bin is enhanced when the strength of momentum and energy transfers increases due to the reduction of extrapolated linear density contrast. Setting the parameters of the dark energy model with momentum coupling according to the observational constraints in \cite{bestfit}, the predicted number counts from the coupled dark energy is larger than the result from eROSITA surveys. The statistical analysis yields a $p$-value of 0.189 for the proposed model relative to $Λ$CDM. Consequently, there is no statistically significant evidence of an improved fit over the standard $Λ$CDM framework based on the eROSITA cluster number counts.

astro-ph.CO↗

arXiv2026

Minimally modified gravity with Laplacian auxiliary constraints and an inflationary realization

We construct a minimally modified gravity theory that propagates only two tensorial gravitational degrees of freedom around a spatially flat Friedmann--Lemaître--Robertson--Walker (FLRW) background and admits a predictive cosmological perturbation theory. We first show that, in the original four-constraint construction, the homogeneous values of the Lagrange multipliers are not fully determined and nevertheless enter the quadratic tensor action, thereby obstructing cosmological predictivity. We remove this ambiguity by coupling the auxiliary constraints to spatial Laplacians of the multipliers. The multiplier sector then drops out of the homogeneous background equations while continuing to constrain the inhomogeneous scalar sector. The tensor dispersion relation generically contains both $k^{2}$ and $k^{4}$ contributions, whereas no propagating gravitational vector or scalar mode is present on the generic branch for which the constraint reduction is nondegenerate. We subsequently study a subclass whose gravitational Hamiltonian density is proportional to the lapse and contains cubic momentum invariants. In this subclass the $k^{4}$ tensor term vanishes and the lapse can be absorbed into a time redefinition at the background level. After coupling a canonical inflaton, the only propagating scalar mode is the inflaton fluctuation, with unit sound speed. For a quadratic potential, departures from general relativity shift the scalar spectral index, while a tensor speed $c_{T}>1$ suppresses the tensor-to-scalar ratio according to $r=16ε_{s}/c_{T}$. The parameter regions compatible with the observational bounds adopted in this work require a superluminal tensor speed; however, very large $c_{T}$ simultaneously reduces the tensor kinetic coefficient and may lower the perturbative cutoff, although determining the actual strong-coupling scale requires a nonlinear analysis.

astro-ph.CO↗

arXiv2025

A Unified Dynamical Systems Framework for Cosmology in $f(Q)$ Gravity: Generic Features Beyond the Coincident Gauge

We present a unified dynamical systems framework for spatially flat FLRW cosmology in $f(Q)$ gravity, covering all three connection branches via a single set of Hubble-normalised variables without fixing $f(Q)$ \textit{a priori}. This connection-agnostic, model-independent approach enables direct comparison across branches and reveals generic structural features that are not apparent in model or connection-specific analyses. Beyond fixed points, we identify invariant submanifolds, model-independent trajectories, and viable phase-space regions common to multiple branches. For a broad class of viable $f(Q)$ models, we find generic de Sitter attractors and matter-dominated points in non-coincident branches, ensuring late-time acceleration without fine-tuning. An invariant submanifold is shown to reproduce $Λ$CDM-like backgrounds despite dynamics distinct from GR, offering a geometric origin for cosmic acceleration detectable only at the perturbation level. On this submanifold, a first integral enables analytic reconstruction of the dynamical connection and uncovers hidden conservation laws. While trivial connections display strong parameter dependence, nontrivial branches often exhibit parameter-independent behaviour. We also analyse the variation of the effective gravitational coupling $κ_{\text{eff}}=\frac{1}{f_Q}$ across branches, providing observational constraints that bridge theory and data. Applying the framework to $f(Q)=αQ+β(-Q)^n$, we recover late-time acceleration and $Λ$CDM-like behaviour without vacuum energy. Finally, we propose a general route for extending dynamical systems analysis to broader classes of $f(Q)$ models using the $m_i$-hierarchy method, which enables closure of the autonomous system for models previously inaccessible to standard approaches.

gr-qc↗

arXiv2025

Reproducing $Λ$CDM-like Solutions in $f(Q)$ Gravity: A Comprehensive Study Across All Connection Branches

Given the remarkable success of the $Λ$CDM model in fitting various cosmological observations, a pertinent question in assessing the phenomenological viability of modified gravity theories is whether they can reproduce an exactly $Λ$CDM-like cosmic background evolution. In this paper, we address this question in the context of $f(Q)$ gravity, where $Q$ denotes the nonmetricity scalar. It is known that there are three possible symmetric teleparallel connection branches that respect the cosmological principles of spatial homogeneity, isotropy, and global spatial flatness. By enforcing a $Λ$CDM-like background evolution via the cosmographic condition $j(z)=1$, where $j$ is the jerk parameter, we reconstruct the $Λ$CDM-mimicking $f(Q)$ theory for each of the three possible connection branches. For the first connection branch, also known as the ``coincident gauge'' in cosmology, we recover the previously known result that a theory of the form $f(Q)=-2Λ+αQ+β\sqrt{-Q}$ can exactly reproduce a $Λ$CDM-like cosmic evolution. Furthermore, we establish that the stability of the $Λ$CDM-like cosmic solution within this reconstructed $f(Q)$, as well as the robustness of the reconstructed $f(Q)$ form with respect to small errors in the astrophysical measurements of the jerk parameter. For the second connection branch, we analytically reconstruct the $Λ$CDM-mimicking $f(Q)$ to be of the form $f(Q)=-2Λ+αQ-βQ^2$. For the third connection branch, we could decouple the evolution equation for the dynamical connection function, which enabled us to perform a numerical reconstruction. Our analysis proves that, at least at the background level, it is possible to obtain $Λ$CDM-mimicking $f(Q)$ models for all the three possible connection branches.

gr-qc↗

arXiv2023

Observational predictions of inflationary model in spatially covariant gravity with two tensorial degrees of freedom for gravity

We study the inflationary model constructed from a Spatially Covariant Gravity (SCG). The Lagrangian for the SCG in our consideration is expressed as the polynomial of irreducible SCG monomials where the total number of derivatives of each monomial is two, and the theory propagates two tensorial degrees of freedom of gravity up to the first order in cosmological perturbations. The condition for having two tensorial degrees of freedom studied earlier in literature for such theories is derived in vacuum. We extend the condition for having two tensorial degrees of freedom to the case where a scalar field is included by imposing a gauge-fixing. We apply the resulting SCG to describe inflationary universe. The observational predictions such as the scalar spectral index and tensor-to-scalar ratio from this model are investigated. We find that the tensor-to-scalar ratio in this model can either be in the order of unity or be small depending on the parameter of the model.

gr-qc↗

arXiv2023

Observational Constraints and Preheating in Cuscuton Inflation

We study cuscuton inflation for the models where the potential of the cuscuton takes quadratic and exponential forms. We find that for the quadratic potential, a scalar spectral index $n_s$ is not affected by cuscuton at the leading order in the slow-roll inflation models. However, a tensor-to-scalar ratio $r$ can be suppressed. For the exponential potential of cuscuton, we find the condition for which the inflation has a graceful exit. Under this condition, the observational predictions in this model differ by a few percent from those found in standard inflation. We also examine the particle production due to parametric resonances in both models. We find that in Minkowski space the stage of parametric resonances can be described by the Mathieu equation. For the case where the cuscuton has quadratic potential, the amplitude of the driving force in the Mathieu equation has a similar form as that in standard inflation. Nevertheless, in the case of exponential potential, the amplitude of the driving force decreases faster than that in the standard case. However, parametric resonances in our models can be sufficiently broad possible for the exponential growth of the number of particles. We briefly discuss the case in which the expansion of space is taken into account.

gr-qc↗

arXiv2022

Cosmic evolution in DHOST theories with scaling solutions

We study cosmic evolution based on the fixed points in the dynamical analysis of the Degenerate Higher-Order Scalar-Tensor (DHOST) theories. We consider the DHOST theory in which the propagation speed of gravitational waves is equal to the speed of light, the tensor perturbations do not decay to dark energy perturbations, and the scaling solutions exist. The scaling fixed point associated with late time acceleration of universe can be either stable or saddle depending on the parameters of the theory. For some ranges of the parameters, this scaling fixed point and field dominated fixed point can be simultaneously stable. Cosmic evolution will reach either the scaling attractor or the field dominated attractor depending on signs of time derivative of the scalar field in the theory during the matter domination. The density parameter of dark matter can be larger than unity before reaching the scaling attractor if the deviation from the Einstein theory of gravity is too large. For this DHOST theory, stabilities of $ϕ$-matter-dominated epoch ($ϕ$MDE) and field dominated solutions are similar to the coupled dark energy models in Einstein gravity even though gravity is described by different theories. In our consideration, the universe can only evolve from the $ϕ$MDE regime to the field dominated regime. The ghost and gradient instabilities up to linear order in cosmological perturbations have been investigated. There is no gradient instability, while the ghost instability can be avoided for some range of the parameters of the model.

astro-ph.CO↗

arXiv2022

Coupled dark energy model inspired from general conformal transformation

We study the coupled dark energy model constructed from the general conformal transformation in which the coefficient of the conformal transformation depends on both the scalar field and its kinetic term. Under this conformal transformation, the action for subclass of Degenerate Higher-Order Scalar-Tensor (DHOST) theories is related to the Einstein-Hilbert action. The evolution of the background universe has the scaling fixed point which corresponds to acceleration of the universe at late time. For the choices of parameters which make the late-time scaling point stable, the fixed point corresponding to $ϕ$-matter-dominated-era ($ϕ$MDE) is a saddle point, and the universe can evolve from radiation dominated epoch through $ϕ$MDE before reaching the scaling point at late time with the cosmological parameters which satisfy the observational bound. During the $ϕ$MDE, the effective equation of state parameter is slightly positive, so that one of possible mechanisms for alleviating the $H_0$ tension can be achieved. In this coupled dark energy model, the effective gravitational coupling for dark matter perturbations on small scales can be smaller than that in the $Λ$CDM model. Therefore a growth rate of the dark matter perturbations is suppressed compare with the $Λ$CDM model, which implies that the $σ_8$ tension could be alleviated.

gr-qc↗

arXiv2021

Inflationary model in minimally modified gravity theories

We have investigated inflationary model constructed from minimally modified gravity (MMG) theories. The MMG theory in the form of $f({\bf H}) \propto {\bf H}^{1+p}$ gravity where, ${\bf H}$ is the Hamiltonian constraint in the Einstein gravity and $p$ is constant, has been studied. An inflation is difficult to be achieved in this theory of gravity unless an additional scalar field playing a role of inflaton is introduced in the model. We have found that the inflaton with exponential potential can drive inflation with graceful exist different from the case of Einstein gravity. The slow-roll parameter for both the exponential and the power-law potentials is inversely proportional to number of e-folding similar to the case of the Einstein gravity. We also have found for the scalar perturbation that the curvature perturbation in the comoving gauge on super Hubble radius scales grows rapidly during inflation unless $p =0$. For the tensor modes, the amplitude of the perturbations is constant on large scales, and sound speed of the perturbations can diviate from unity and can vary with time depending on the form of $f({\bf H})$.

astro-ph.CO↗

arXiv2019

Generalized Conformal Transformation and Inflationary Attractors

We investigate the inflationary attractors in models of inflation inspired from general conformal transformation of general scalar-tensor theories to the Einstein frame. The coefficient of the conformal transformation in our study depends on both the scalar field and its kinetic term. Therefore the relevant scalar-tensor theories display the subset of the class I of the degenerate higher-order scalar-tensor theories in which both the scalar field and its kinetic term can non-minimally couple to gravity. We find that if the conformal coefficient $Ω$ takes a multiplicative form such that $Ω\equiv w(ϕ)W(X)$ where $X$ is the kinetic term of the field $ϕ$, the theoretical predictions of the proposed model can have usual universal attractor independent of any functions of $W(X)$. For the case where $Ω$ takes an additive form, such that $Ω\equiv w(ϕ) + k(ϕ) Ξ(X)$, we find that there are new $ξ$ attractors in addition to the universal ones. We analyze the inflationary observables of these models and compare them to the latest constraints from the Planck collaboration. We find that the observable quantities associated to these new $ξ$ attractors do not satisfy the constraints from Planck data at a strong coupling limit.

gr-qc↗

arXiv2018

Squeezed bispectrum from multi-field inflation with curved field space metric

We investigate influences of the curved field-space metric of multi-field inflationary models on the squeezed bispectrum. The reduced bispectrum in squeezed limit is computed using the δN formalism. The calculation is performed under the slow-roll approximation and assumption that field derivative of the field-space metric is sufficiently small such that the contributions from Riemann tensor of the field-space can be approximately ignored. Based on these approximations, We compute the analytic expressions for the reduced bispectrum in squeezed limit, and find that, for such a nearly flat field-space metric, the field dependence of the metric can significantly alter both amplitude and shape of the reduced bispectrum. The reduced bispectrum from this nearly flat field-space metric can lead to spectral index of the halo bias which amplitude is 2 -- 4 times larger than that from the flat field-space model. This modification of the spectral index of the halo bias due to the curved field-space metric could leave observable imprints in future galaxy surveys.

astro-ph.CO↗

arXiv2018

Spherical collapse and cluster number counts in dark energy models disformally coupled to dark matter

We investigate the effects of a disformal coupling between dark energy and dark matter in the predictions of the spherical collapse and its signatures in galaxy cluster number counts. We find that the disformal coupling has no significant effects on spherical collapse at high redshifts, and in particular during matter domination epoch. However, at lower redshifts, the extrapolated linear density contrast at collapse close to redshift $z \lesssim 1$ and overdensity at virialization can be strongly suppressed by a disformal coupling between dark energy and dark matter. We also find that disformal coupling can have different imprints on cluster number counts compared with conformal coupling, such that the disformal coupling can strongly suppress the predicted number of clusters per redshift interval at $z > 0.1$ while enhance the number of cluster at $z < 0.05$. Using the specifications of eROSITA survey, we find that the disformal coupling between dark energy and dark matter can be tightly constrained by cluster number counts.

astro-ph.CO↗

arXiv2017

Preheating in an inflationary model with disformal coupling

In this work, we investigate the preheating mechanism in a disformally-coupled inflationary model where the scalar field $ϕ$ (which is the inflaton field) naturally coupled to another matter field $χ$ induced by the disformal transformation. In the present scenario, novel derivative interactions mixing the kinetic terms of the two fields emerge inherently. We start by deriving the evolution of the background system when the back reaction on the background field is neglected. We examine the particle production due to parametric resonances in the models and find in Minkowski space that the stage of parametric resonances can be described by the Mathieu equation. Interestingly, we discover that broad resonances in our models can be typically achieved. Finally, we compare our results with previously studied model with derivative couplings.

hep-ph↗

arXiv2017

Vainshtein mechanism in general purely disformal gravity theory

We study a theory of gravity in which the action is a result from the general purely disformal transformation on the Einstein-Hilbert action. This theory is a sub-class of GLPV theory which is the the generalization of covariant Galileon. Nevertheless, we find that the self accelerating solution for the background universe disappears in this theory. We also find that, for this theory, the Vainshtein mechanism is absent. However, the Vainshtein mechanism is not necessary for this theory, because this theory can nearly mimic the Einstein theory of gravity at all scales inside the Huble radius without this mechanism.

gr-qc↗

arXiv2017

Dynamics of the universe with disformal coupling between the dark sectors

We use the dynamical analysis to study the evolution of the universe at late time for the model in which the interaction between dark energy and dark matter is inspired by disformal transformation. We extend the analysis in the existing literature by supposing that the disformal coefficient depends both on the scalar field and its kinetic terms. We find that the dependence of the disformal coefficient on the kinetic term of scalar field leads to two classes of the scaling fixed points that can describe the acceleration of the universe at late time. The first class exists only for the case where the disformal coefficient depends on the kinetic terms. The fixed points in this class are saddle points unless the slope of the conformal coefficient is sufficiently large. The second class can be viewed as the generalization of the fixed points studied in the literature. According to the stability analysis of these fixed points, we find that the stable fixed point can take two different physically relevant values for the same value of the parameters of the model. These different values of the fixed points can be reached for different initial conditions for the equation of state parameter of dark energy. We also discuss the situations in which this feature disappears.

gr-qc↗

arXiv2014

Large tensor-to-scalar ratio from Composite Inflation

The claimed detection of the BICEP2 experiment on the primordial B-mode of cosmic microwave background polarization suggests that cosmic inflation possibly takes place at the energy around the grand unified theory scale given a constraint on the tensor-to-scalar ratio. i.e., $r\simeq 0.20$. In this report, we revisit single-field (slow-roll) composite inflation and show that, with the proper choice of parameters and sizeable number of e-foldings, a large tensor-to-scalar ratio consistent with the recent BICEP2 results can be significantly produced with regard to the composite paradigms.

astro-ph.CO↗

arXiv2014

Composite Inflation confronts BICEP2 and PLANCK

We examine observational constraints on single-field inflation in which the inflaton is a composite field stemming from a four-dimensional strongly interacting field theory. We confront the predictions with the Planck and very recent BICEP2 data. In the large non-minimal coupling regions, we discover for MCI model that the predictions lie well inside the joint $68\%$ CL for the Planck data, but is in tension with the recent BICEP2 observations. In the case of the GI model, the predictions satisfy the Planck results. However, this model can produce a large tensor-to-scalar ratio consistent with the recent BICEP2 observations if the number of e-foldings is slightly smaller than the range commonly used. For a super Yang-Mills paradigm, we discover that the predictions satisfy the Planck data, and surprisingly a large tensor-to-scalar ratio consistent with the BICEP2 results can also be produced for an acceptable range of the number of e-foldings and of the confining scale. In the small non-minimal coupling regions, all of the models can satisfy the BICEP2 results. However, the predictions of GI and SgbI models cannot satisfy the observational bound on the amplitude of the curvature perturbation launched by Planck, and the techni-inflaton self-coupling in the MCI model is constrained to be extremely small.

hep-ph↗